1. Field of the Invention
This invention relates to a flow amplified control system and more particularly to a load responsive flow amplified control system in which the ratio of amplification between the operating fluid circuit and the control fluid circuit can be varied throughout a range of operation.
2. Description of the Prior Art
Flow amplified steering systems are sometimes used on earthmoving and industrial type mobile vehicles. Such systems normally comprise a pilot or control fluid circuit and a main steering or operating fluid circuit. The main steering circuit usually includes a pilot operated directional control valve which controls the flow of pressurized fluid from a source pump to and from one or more bidirectional steering motors. The basic component of the pilot circuit is normally a hand metering unit which directs a metered volume of fluid to a selected end of the directional control valve in response to rotation of a steering wheel connected to the hand metering unit. The degree of movement of the directional control valve is controlled by the degree of rotation of the steering wheel. If the steering wheel is rotated slowly a low volume of pilot fluid will be directed to the directional control valve resulting in low spool movement. A faster rotation of the steering wheel directs a larger volume of pilot fluid to the directional control valve resulting in greater movement of the directional control valve. The volume of fluid directed to the steering motor is many times greater than the volume of pilot fluid directed to the directional control valve and is dependent upon the degree of the movement of the directional control valve and thus the degree and speed of rotation of the steering wheel. One such system is disclosed in U.S. Pat. No. 3,566,749 dated Mar. 2, 1971 and issued to Allen, et al.
One version of the above described steering system used commercially for many years includes a variable flow amplification ratio throughout the operating range wherein the amplification ratio at the initial opening of the directional control valve is less than the amplification ratio at the full open position of the directional control valve.
Attempts have been made to increase the overall efficiency of some of the flow amplified steering systems by combining the pilot fluid used to move the directional control valve with the main operating fluid being directed to the steering motors. One of the problems with such systems is that heretofore combining the pilot fluid and the main supply fluid has been done through fixed-sized orifices. This essentially provides a predetermined ratio of flow amplification throughout the total operating range thereby limiting the versatility of the steering system. In many uses it is desirable to vary the ratio of flow amplification throughout the total operating range. Another problem with such systems is that they require one spool for determining the flow amplification ratio and another spool for determining the direction of the fluid flow. Both of the spools are sized to handle the full volume of the operating circuit such that the size and complexity of the valve increases.
The present invention is directed to overcoming one or more of the problems as set forth above.